Urban watershed redevelopment: Design scenarios for reducing phosphorus pollution from stormwater in Boston's Charles River basin, USA

This study examines potential improvements to water quality in urban rivers resulting from the use of stormwater treatment landscapes---specifically, detention ponds and biofiltration cells---on a watershed scale. The 2007 Charles River Phosphorus TMDL requires 65% reduction of phosphorus (P) loading to the river from urban land uses. Two ∼200-acre urban case study sites in Boston's lower Charles River basin were modeled, using WinSLAMM (the Source Loading and Management Model for Windows) to characterize the P-reduction of various detention pond and biofiltration cell (biofilter) arrangements within the sites.;The research goals were to evaluate the effects on water quality (P-reduction) associated with (A) percent cover of a site by detention ponds or biofilters and (B) configurations (numbers/sizes) of ponds/biofilters, and to (C) develop and analyze alternative conceptual stormwater plans for the study sites, based on the modeling results. Stormwater landscape configurations ranged from "consolidated" to "dispersed" across the study sites.;Modeling indicated that both placement and area of stormwater treatment landscapes are important. With 10% pond coverage across either site, the 65% P-removal target was only met when runoff from an entire site entered detention ponds. Similar P-reduction levels were achieved using few (4-5) large ponds (consolidated arrangements) or many (29-40) smaller ponds (dispersed arrangements). A single very large pond placed at either site's outfall achieved the P-removal goal with only 5% total pond coverage, i.e. less surface area than other pond arrangements. Modeling for numerous tiny biofilters on each site showed greater P-removal when no "underdrains" were present than when these drainage pipes were used. Without underdrains, P-removal differed little between the two sites and between using larger (435 ft2) and smaller (200 ft2) biofilters. The modeling results were meshed with on-site land use patterns and societal goals (e.g., pedestrian circulation, recreation, aesthetics, existing redevelopment proposals) to produce and compare several alternative stormwater plans for each site.;This research demonstrates that the Charles River Phosphorus TMDL is attainable with realistic allocations of area for stormwater treatment. Indeed, stormwater landscape designs in urban settings can create community parks and pedestrian networks, while also improving downstream water quality.